A shit smear on your asscrack or in your drawers that includes a non-shit chunk. Said chunk can consist of chile seeds, any tree nut, undigested items (including paper, metal and wood), Capt'n Crunch and more. This does not include klingons.

I hate doing laundry because dad always has chumpstains in his underpants.

Dude, I went to eat Erin's pussy and the bitch had a gnar chumpstain! She must wipe from back to front.